Detecting gases is useful for a variety of reasons. With respect to environmental concerns, an apparatus for detecting pollution or industrial emission is beneficial to help limit such contaminants entering water systems or the atmosphere. A gas detection unit may also be used for detecting the presence of dangerous chemical compounds, such as carbon monoxide, in a mixture of gases. In the medical field, a gas detection unit may be used for detecting a particular gas in equipment, such as an oxygen inhalation machine, for alerting staff as to the amount of oxygen remaining in the reservoir or given to the patient.
Known methods and apparatuses have been developed to detect the presence of gases. Typical systems include gas chromatography, ion chromatography, electrolytic conductivity detection, and conductometric measurement. However, these manners for detecting gases have generally been expensive, cumbersome, or shown to have low sensitivities and slower response times. In situations where a generally quick response time may be desired, such as detecting toxic gases or a lack of oxygen in an oxygen inhalation machine, gas detection systems having enhanced abilities to quickly detect particular gases are usually favorable.
Electrochemical sensors were provided to overcome these limitations. Electrochemical sensors typically provide signals which tend to exhibit acceptable sensitivity and usually have quick response times relative to gas chromatography, ion chromatograph, and electrolytic conductivity detection systems.
Other electrochemical gas sensors typically include metal layers or electrodes in contact with and beneath an electrolytic film of, for example, Nafion or Teflon. However, because the gas usually needs to diffuse through the ionic medium before reaching the sensing electrode, the response time may be negatively affected.
Recently, planar thin film sensors have been developed by constructing three planar electrodes on an insulating substrate and covering them with a thin polymer electrolyte, such as Nafion. J. A. Cox and K. S. Alber, Amperometric Gas Phase Sensor for the Determination of Ammonia in a Solid State Cell Prepared by a Sol-Gel Process, 143, No. 7 J. Electrochem. Soc. L126-L128 (1996) developed a solid state cell in which microelectrode arrays were coated with a film of vanadium oxide xerogel for detection of ammonia. However, this film needs to be soaked in an electrolyte solution in order to provide ionic conductivity. These methodologies, in which a planar substrate with metal electrodes is covered with a thin film of solid state electrolytic material, are suitable for automated mass production, but they have longer response times since gas needs to diffuse through a relatively thick film of electrolyte.
As shown electrochemical gas sensor 10 includes substrate 11, electrode 3, and ionomer membrane 5. Gas enters and exits sensor 10 through the inlet and outlet as shown. A portion of the gas entering sensor 10 diffuses through diffusion hole 20 and contacts electrode 3, which detects the type of gas present in sensor 10.
To enhance sensitivity to sensor 10, a reservoir 9 is provided containing electrolyte solution to wet ionomer membrane 5. As shown, reservoir 9 and, therefore, the electrolyte solution is in contact with ionomer membrane 5. Because reservoir 9 is located on a same side of ionomer membrane 5 as diffusion hole 20, a length of diffusion hole is typically at least as long as a height of reservoir 9.
What is desired, therefore, is an electrochemical sensor that overcomes the limitations of the prior art to provide a further improved response time. What is also desired is an electrochemical sensor having a wetted electrolytic medium to maintain sensitivity. A further desire is to provide an electrochemical sensor having a diffusion control passage for controlling the flow of gas leading to the sensing electrode.